Radiation therapy, or radiotherapy, uses high-energy radiation to destroy target cells. In external-beam radiation therapy, photon beams emitted by radiation treatment machine, such as a linear accelerator, are focused on a target volume within the body. Circular collimators, often referred to as “cones,” have been used to limit the breadth of the radiation emitted by the linear accelerator so as to limit the radiation exposure of non-targeted patient tissue. Such cones are normally attached to the linear accelerator immediately prior to radiotherapy treatment. Testing is required to ensure that the cone is aligned with the target volume with high accuracy. In addition, the linear accelerator must be properly configured to use the cone. Unfortunately, such testing and configuring is time consuming and errors in these processes can result in the misadministration of radiation to the patient.
Later model linear accelerators are typically equipped with built-in multileaf collimators (MLCs) that ameliorate the problems of cones and their use. MLCs comprise many thin “leaves” of a high atomic numbered material, such as tungsten, that can be independently linearly moved into and out of the path of a radiation beam in order to block it. Accordingly, MLCs can be used to conformally shape radiotherapy treatment beams.
While MLCs overcome the drawbacks of cones, the resolution of an MLC is typically insufficient for very small target volumes, such as those approximately 5 mm or less in diameter. In addition, configuration of the MLC is typically patient-specific and the calculation and measurement of the resulting dose distribution is complicated and fraught with difficulties that can result in errors.
From the above discussion, it can be appreciated that it would be desirable to have an alternative way to deliver very small radiation dose distributions using a radiation treatment machine equipped with an MLC.